Color image sensor on transparent substrate and method for making same

ABSTRACT

The invention concerns a color image sensor that can be used to make a miniature camera, and a corresponding method for making this sensor. 
     The image sensor comprises a transparent substrate ( 40 ) on the upper part of which are superimposed, successively, a mosaic of color filters ( 18 ), a very thin silicon layer ( 30 ) comprising photosensitive zones, and a stack of conductive layers ( 14 ) and insulating layers ( 16 ) defining image detection circuits enabling the collection of the electrical charges generated by the illumination of the photosensitive zones through the transparent substrate. The manufacturing method consists in producing the photosensitive circuits on a silicon wafer, transferring said wafer on to a temporary substrate, thinning the wafer down to a thickness of about three to 30 micrometers, depositing color filters on the surface of the remaining silicon layer and transferring the structure to a permanent transparent substrate and eliminating the temporary substrate.

The invention relates to electronic image sensors, and especially tovery small-sized sensors with dimensions that enable the making ofminiature cameras such as those that are to be incorporated into aportable telephone.

Apart from great compactness, the image sensor should have highsensitivity under weak light and high colorimetrical performance.

Furthermore, the entire camera needs to be made by the most economicalmethods possible so as not to make the apparatus prohibitively costly.

To achieve this result, it is sought firstly to make the image sensorand the electronic processing circuits if possible on a same siliconsubstrate and secondly, as far as possible, to carry out the depositionof the different layers, the etching operations, the heat-processingoperations etc. collectively on a silicon wafer comprising manyidentical sensors, and then dice the wafer into individual sensors.

However, the methods hitherto proposed for making color image sensorsand the structures of these sensors are not entirely satisfactory fromthis viewpoint. The methods of manufacture are not industriallyefficient; they remain far too costly and their efficiency is far toolow for large-scale manufacturing applications, or else the performanceof the image sensor is not high enough.

The present invention proposes a method of manufacture and acorresponding image sensor that minimizes the costs of manufacture whilepresenting excellent quality and especially compactness, highsensitivity and high colorimetrical performance.

To this end, the invention propose a method for making an image sensor,comprising:

-   -   the formation, on the front face of a semiconductive wafer, of a        series of active zones comprising image detection circuits and        each corresponding to a respective image sensor, each active        zone being surrounded by input/output pads,    -   the transfer of the wafer by its front face against the front        face of a temporary supporting substrate,    -   the elimination of the major part of the thickness of the        semiconductive wafer, leaving a very fine semiconductive layer        on the substrate, this fine semiconductive layer comprising the        image detection circuits,    -   this method being characterized in that    -   firstly, layers of color filters are deposited and then etched        on the semiconductive layer thus thinned,    -   secondly, after the etching of the color filters, the entire        temporary substrate and wafer are transferred to a permanent,        transparent substrate applied on the side of the temporary        substrate that bears the color filters,    -   then, at least the major part of the temporary substrate is        removed, to enable easy access to the input/output pads,    -   and finally, the substrate is diced into individual sensors.

The semiconductive material of the thinned layer is preferably amonocrystalline material, and especially silicon for the most usualapplications in visible light.

The temporary substrate may be entirely removed, baring the input/outputpads to which the outward connections of the sensor may then beconnected. But it is also possible to remove it only partially, leavinga thin layer that protects the semiconductive wafer. In this case,apertures need to be formed in this fine layer to access theinput/output pads.

Preferably, the active zones comprise a matrix of photosensitiveelements as well as control circuits of the matrix and associatedimage-processing circuits receiving signals coming from thephotosensitive elements of the active area. The circuits thus associatedwith the matrix are preferably masked against light by a layer ofaluminum, only the matrix being exposed to light. This aluminum layer isformed on the transparent substrate.

The transfer of the semiconductive wafer to the temporary substrate canbe done by gluing, classic soldering, anodic bonding or by simplemolecular adhesion (i.e. through the very great force of contact betweentwo surfaces having great planeity). The transfer from the temporarysubstrate to the permanent substrate will preferably be done by bondingor by molecular adhesion.

The thinning of the semiconductive wafer after transfer to the substrateand before the deposition of the filters can be done in many differentways: thinning by lapping, chemical thinning, a combination of bothtypes of thinning (firstly mechanical thinning and then chemicalfinishing or else mechanical machining in the presence of chemicals).The thinning can also be done by a preliminary embrittlement of thewafer at the desired dicing level, in particular by in-depth hydrogenimplantation in the desired dicing plane. In this case, the hydrogenimplantation is done at a shallow depth in the semiconductive waferbefore the transfer of the wafer to the substrate. The thinning is thendone by heat processing which dissociates the wafer at the level of theimplanted dicing plane, leaving a thin semiconductive layer in contactwith the substrate.

The very great thinning of the wafer reduces its thickness from severalhundreds of micrometers before transfer to the substrate to 3 to 20micrometers after transfer to the substrate. Thinning is a major factorin the quality of the sensors since it improves colorimetricalperformance and sensitivity. With non-thinned sensors, illuminated bythe side in which there are formed the numerous insulating andconductive layers that serve to define the image detection circuits, thelight that has crossed a color filter is scattered on photosensitivedots corresponding to different colors, lowering colorimetricalperformance. Furthermore, the sensitivity of a thin sensor is improvedbecause the photons reach a wider silicon region than in the case of thenon-thinned sensors, since they are not stopped by the metal layerswhich are opaque and take up a large part of the surface areacorresponding to each photosensitive dot.

It will be understood that the thinning, however, complicates theproblems of manufacture because, after thinning, the silicon loses itsrigidity and becomes very brittle, and that, furthermore, there arisesthe problem of connecting the image detection circuits with theexterior. The solution of the invention mitigates this difficulty andenables the making of the image sensors with high efficiency.

In the permanent sensor, the light is received through the transparentpermanent substrate, the connection pads being located on the otherside, thus enabling the sensor to be mounted by the flip-chip technique(in which the chip is upside down with the connection pads against theprinted circuit board). The light losses through the transparentsubstrate (made of glass or plastic) are low.

The permanent substrate and the silicon layer are in close contact andthe active circuit elements of the wafer are therefore well protected.

For example, the thickness of the permanent substrate is about 500micrometers for a substrate with a diameter of 15 to 20 centimeters; thethickness of the silicon wafer ranges from 500 to 1000 micrometersbefore thinning (with a diameter of 15 to 30 centimeters), and then from3 to 20 micrometers after thinning.

Planarization layers, made of polyimide for example, may be deposited onthe silicon wafer before transfer to the intermediate substrate andbefore the transfer of the intermediate substrate to the permanentsubstrate.

It must be noted that the intermediate substrate in certain cases can bere-used from one manufactured batch to another.

An object of the invention therefore is an image sensor comprising atransparent substrate, on the upper part of which the following aresuperimposed successively a mosaic of color filters, a very thinmonocrystalline semiconductive layer (with a maximum thickness of sometens of micrometers) (30) in which there is formed a matrix array ofphotosensitive zones, and a stack of insulating and conductive layersenabling the collection of the electrical charges generated by theillumination of the photosensitive zones through the transparentsubstrate, so that the light passes, successively, through thetransparent substrate, then the color filters, then the photosensitivesemiconductive zones, and then the stack of insulating and conductivelayers, without encountering a system of conductive layers, beforereaching the array of photosensitive zones.

The transparent substrate is preferably made of glass or plastic but mayalso be made of ceramic or of crystalline material.

Other features and advantages of the invention shall appear from thefollowing detailed description, made with reference to the appendeddrawings, of which:

FIG. 1 shows the structure of an image sensor made on a silicon waferbefore the positioning of color filters;

FIG. 2 shows the operation of the transfer of the silicon wafer by itsfront face to a temporary substrate;

FIG. 3 shows the temporary substrate with the silicon wafer afterthinning of the wafer;

FIG. 4 shows the temporary substrate, bearing the thinned silicon layeron which a mosaic of color filters has been deposited;

FIG. 5 shows the permanent substrate to which the temporary substrate istransferred by its face bearing the colored filters;

FIG. 6 shows the permanent substrate after the elimination of thetotality of the thickness of the temporary substrate;

FIG. 7 shows an alternative embodiment from which the major part but notthe totality of the temporary substrate has been removed, and in whichaccess apertures to contacts have been formed.

FIG. 1 shows the general structure of the silicon wafer on which classictechniques have been used to make the image detection circuits of amultiplicity of image sensors.

The silicon wafer 10 has a thickness of several hundreds of micrometers,for a diameter of 150 to 300 millimeters.

The image detection circuits (the matrix of photosensitive dots,transistors and interconnections) are fabricated on one face of thesilicon wafer, which may be called the front face and is the upper facein FIG. 1. Fabrication implies, firstly, various operations of diffusionand implantation in the silicon, from the upper face of the wafer, toform especially photosensitive zones 12, and, secondly, successiveoperations for the deposition and etching of conductive layers 14 andinsulating layers 16 forming a stack on top of the photosensitive zones12. The insulating and conductive layers form part of the imagedetection circuits and enable the collection of electrical chargesgenerated in the photosensitive zones by an image projected on thesensor.

One of the conductive layers 14, in principle the layer deposited last,serves to form input/output pads of each individual sensor (the padscannot be seen in FIG. 1) around the active zone comprising the matrixof photosensitive dots.

If the sensor were to be made by means of a classic technology, then amosaic of color filters would be deposited on the surface of the wafer.

According to the invention, no color filters are deposited at this stagebut the wafer is transferred by its front face to a temporary substrate20 (FIG. 2). The temporary substrate 20 is a wafer having the samediameter as the wafer 10 and a similar thickness to ensure the rigidityof the structure while it is being made. It may furthermore beconstituted by another silicon wafer. The transfer can be done after thedeposition of a planarization layer serving to fill the relief featurescreated on the front face of the silicon wafer by the operations ofdeposition and etching of the stack of conductive and insulating layers.This planarization layer does not need to be transparent

FIG. 2 represents the structure on a smaller scale than that of FIG. 1in order to show the entire individual sensor comprising an active zoneZA and connection pads 22 around the active zone ZA. The pads 22, incontact with a conductive layer 14 or forming part of the layer 14, arepreferably flush with the interface between the two wafers 10 and 20; ifa planarization layer has been deposited, it is preferably ensured thatit does not cover the pads 22. However, if the pads are covered by theplanarization layer, it will subsequently be seen that apertures can inany case be made to access these pads at the end of the fabricationprocess.

The transfer of the silicon wafer to the supporting wafer 20 can be doneby several means. The simplest means could be quite simply that ofholding the wafer by molecular adhesion, since the great planeity of thesurfaces in contact generates very high contact forces. Gluing is alsopossible.

After the silicon wafer has been transferred by the front face to thesupporting wafer, the major part of the thickness of the silicon waferis eliminated so as to leave only a thickness of about 8 to 30micrometers, including the thickness of the stack of layers. Whatremains of the silicon wafer is no more than a superimposition of a fewmicrometers (5 to 10 micrometers for example) for the stack of layers14, 16 and about three to 20 micrometers for the remaining siliconthickness, including the photosensitive areas 12. The remainingthickness is that of the layer 30 of FIG. 3 containing thephotosensitive zones 12 of FIG. 1.

The thinning operation can be done by mechanical machining (lapping)terminated by chemical machining, or by mechanical/chemical machining,or by chemical machining only, or again by a particular method ofseparation necessitating a preliminary implantation of an embrittlingimpurity in the plane that will demarcate the thinned silicon layer.

In the case of this separation by implantation of impurities, theimplantation must be done before the transfer of the silicon wafer tothe supporting wafer. Indeed, the implantation is done by the front faceof the silicon wafer, throughout the surface of the wafer and at a depththat will define the dicing plane. The preliminary implantation ispreferably hydrogen implantation. It can be done at various stages ofthe making of the wafer, but the separation of the thickness of thewafer along the implanted dicing plane can be done only when the siliconwafer has been attached to the supporting wafer.

The upper surface of the thinned silicon layer 30 can be processed (finelapping, chemical cleaning, mechanical/chemical polishing, etc.) inorder to eliminate the surface defects, after which the color filterscan be deposited and etched, leading to a multiple-sensor wafer whosegeneral structure is that of FIG. 2.

A mosaic of color filters 18 is then deposited on the surface of thelayer 30 (FIG. 4). If desired, one or more additional layers can bedeposited before the deposition of the color filters, especiallypassivation layers, anti-reflection layers and other layers, for examplelayers needed for the electrical activation of the doped silicon layers(electrical polarization layers).

If necessary, a planarization layer 24 is deposited on the mosaic offilters. It must be transparent if it covers the filters. The temporarysubstrate 20 is then transferred, by its front face bearing the colorfilters, to a permanent transparent substrate 40 (made of glass orplastic), in the form of a wafer having a diameter identical to that ofthe temporary substrate and the initial silicon wafer. The thickness ofthe permanent substrate is equal to a few hundreds of micrometers atleast, to make it possible to ensure the rigidity of the structureduring fabrication. (FIG. 5).

The transfer of the temporary substrate to the permanent substrate isdone by gluing (with transparent glue) or by molecular adhesion.

The major part or even the totality of the temporary substrate 20 iseliminated by mechanical and/or chemical means, or by embrittlement byhydrogen implantation for example as already explained. In this case, topartially remove the substrate 20, the hydrogen implantation in thesupporting wafer 20 must be done prior to the first transfer of thesilicon wafer to the wafer 20. This implies that, between the transferto the wafer 20 and the transfer to the substrate 80, no operation isperformed at temperatures liable to cause a break at the hydrogenimplantation plane.

In the case shown in FIG. 6, the substrate 20 is totally eliminateduntil the connection pads 22 are made flush with the surface of thestructure.

In the case of FIG. 7, the elimination of the substrate 20 is onlypartial. There remains a small thickness (some micrometers at most ifpossible) in which chemical corrosion or other means will be used toform apertures 70 to open up regions of access to the connection pads22.

The connection pads in the case of FIG. 7 may be used for a“wire-bonding” type of connection with a printed circuit board. Sincethe light must penetrate from the transparent permanent substrate 40side, the printed circuit board must then be open so as to be facing thephotosensitive active zone of the sensor.

In case of FIG. 6, the connection pads 22 are flush with the level ofthe upper surface of the image sensor. They may be used either for a“wire-bonding” type connection or for a “flip-chip” type of connection(the chip being placed upside down with the connection pads against thecorresponding pads of the printed circuit board). In this case, thesensor is illuminated through the top of the printed circuit board.

If it were desired all the same to use a flip-chip type of assembly forthe sensor shown in FIG. 7, in which the connection pads 22 are pushedinto the apertures 70, the following procedure would be used: anadditional metallization would be deposited and etched, thismetallization resting on the outer surface of the structure (i.e. on theupper surface of the remaining portion of substrate 20 in whichapertures 70 have been formed) as well as at the bottom of the apertures70. The external connection pads of the structure would then be formedoutside the apertures 70.

In these different embodiments, the structure formed on the substrate 40may be tested on the wafer by means of the connection pads. The test maybe performed in the presence of light, image patterns, etc.

The structure is diced into individual sensors for packaging only at theend of this fabrication process.

The permanent substrate, applied closely against the thinned siliconlayer bearing the color filters, protects both the filters and thesilicon.

1. A method for making an image sensor, comprising the steps of:forming, on the front face of a semiconductive wafer, a series of activezones comprising image detection circuits each corresponding to arespective image sensor, each active zone being surrounded byinput/output pads, then, transferring of the wafer by its front faceagainst the front face of a temporary supporting substrate; then,eliminating a major part of the thickness of the semiconductive wafer,leaving a very fine semiconductive layer on the substrate, this finesemiconductive layer comprising the image detection circuits; then,layers of color filters are deposited and then etched on thesemiconductive layer thus thinned, then, after the etching of the colorfilters, the temporary substrate is are transferred to a transparent,permanent substrate applied on the side of the temporary substrate thatbears the color filters, then, at least the major part of the temporarysubstrate is removed, to enable easy access to the input/output pads,and finally, the substrate is diced into individual sensors.
 2. Themethod according to claim 1, wherein the temporary substrate is entirelyremoved, baring the input/output pads.
 3. The method according to claim1, wherein the temporary substrate is partially removed, leaving a thinlayer that protects the semiconductive wafer, and apertures are formedin this fine layer to access the input/output pads.
 4. The methodaccording to claim 1, wherein the semiconductive layer after thinninghas a thickness of about 3 to 20 micrometers.
 5. The method according toclaim 1, wherein a planarization layer is deposited on thesemiconductive wafer before transfer to the temporary substrate.
 6. Themethod according to claim 1, wherein a transparent planarization layeris deposited on the thinned semiconductive layer before transfer fromthe intermediate substrate to the permanent substrate.
 7. An imagesensor comprising: a transparent substrate on the upper part of whichthe following are superimposed, successively, a mosaic of color filters,a very thin monocrystalline semiconductive layer in which there isformed a matrix array of photosensitive zones, and a stack of conductivelayers and insulating layers enabling the collection of the electricalcharges generated by the illumination of the photosensitive zonesthrough the transparent substrate, so that the light passes,successively, through the transparent substrate, then the color filters,then the photosensitive semiconductive zones, and then the stack ofinsulating and conductive layers, without encountering a system ofconductive layers, before reaching the array of photosensitive zones. 8.The image sensor according to claim 7, wherein the transparent substrateis made of glass, plastic or crystalline material.
 9. The methodaccording to claim 2, wherein a planarization layer is deposited on thesemiconductive wafer before transfer to the temporary substrate.
 10. Themethod according to claim 3, wherein a planarization layer is depositedon the semiconductive wafer before transfer to the temporary substrate.11. The method according to claim 4, wherein a planarization layer isdeposited on the semiconductive wafer before transfer to the temporarysubstrate.
 12. The method according to claim 2, wherein a transparentplanarization layer is deposited on the thinned semiconductive layerbefore transfer from the intermediate substrate to the permanentsubstrate.
 13. The method according to claim 3, wherein a transparentplanarization layer is deposited on the thinned semiconductive layerbefore transfer from the intermediate substrate to the permanentsubstrate.
 14. The method according to claim 4, wherein a transparentplanarization layer is deposited on the thinned semiconductive layerbefore transfer from the intermediate substrate to the permanentsubstrate.
 15. The method according to claim 5, wherein a transparentplanarization layer is deposited on the thinned semiconductive layerbefore transfer from the intermediate substrate to the permanentsubstrate.